When completing and prior to starting production in petroleum wells, it is sometimes necessary or desirable to carry out hydraulic fracturing operations (commonly referred to as ‘fracking’). In such fracking operations, the well is pressurized with a hydraulic fluid, so as to fracture the formation and improve the flow conditions for the hydrocarbons.
It is preferable to carry out fracking operations individually and sequentially for different sections of the well; this avoids the need to pressurize the entire well and thus reduces the pumping capacity required for the operation. This can be done by arranging packer elements at longitudinal intervals on the outside of the production pipe that is led into the well at the reservoir. The packer elements, for example made from a rubber material, are arranged to swell up against the well casing or formation and form a seal in the annulus between the casing and the production pipe. By using several such elements, the well is divided into a number of closed zones between these seals.
A number of valves are arranged in the production pipe, corresponding to each zone. Commonly, each valve is opened by dropping a ball (or a different type of activation element) down into the production pipe, which then stops in a seat in the valve. The pressure is then increased above the ball and a slide or casing mechanism is pushed down to open the valve. Normally this is achieved in that the valve that is placed uppermost in the production pipe has a ball seat with a large diameter, with the diameter of the ball seats of the other valves decreasing successively down the well. By first letting down a small ball in the pipe, one will then pass through all the upper valves and get the ball landed on the seat in the lowermost valve. Thus, one can choose the correct valve according to the size of the ball, in order to start the fracturing in a desired zone.
One limitation of this system is that it requires ball seats with a large diameter for the uppermost valves, and successively smaller and smaller ball seats as one proceeds down the well. If using a large number of zones, which is desirable in long wells or to obtain better fracturing performance, a large number of valves is required. Since the inner diameter of the production pipe is limited, this necessitates small increments between the size of the valve seats, and very small ball seats in the lowermost valves. This makes the process more prone to errors (e.g., that a ball gets stuck in the wrong valve or that the wrong valve is activated) and is undesirable during production from the well, when such valve seats create a flow restriction for the hydrocarbons flowing upwards in the production pipe. Moreover, the valve seats create obstacles if a tool, for example a wireline tool, is later to be used in the production string, for example for well intervention purposes.
Some prior art solutions have aimed at developing systems with ball-activated valves and where all valves can be activated by a ball of the same size. These are, however, generally mechanically complex and thus more expensive, and also prone to failures. Other alternatives also exist, such as using a wireline tool to activate the valves, however this is laborious and also carries a risk of errors, for example that the wireline tool gets stuck in the well.
Documents which can be useful for understanding the background include U.S. Pat. No. 9,004,180; WO 2016/028154; WO 2015/134014; US 2012/0085548; US 2011/0203800; WO 2010/127457; US 2014/151054; US 2011/030976; U.S. Pat. No. 8,783,365; and WO 2016/003759.
The present invention has the objective to provide tools and equipment suitable for use in hydraulic fracturing operations and associated methods, which provide advantages over known solutions and techniques in relation to the aspects mentioned above or others.